1. Field of the Invention
The present invention relates to an optical disk device, particularly to an optical disk device to mitigate the influence of spherical aberration that results from individual differences of optical disks such as thickness variation in a cover layer.
2. Description of Prior Art
Storage density of an optical disk is limited mostly by the spot size of beam for recording and reproducing; λ/NA (λ: wavelength, NA: numerical aperture of an objective lens). In order to increase the capacity of the optical disk, therefore, it is necessary to shorten the wavelength and increase the numerical aperture.
Short wavelength lasers such as blue-violet laser and objective lenses with large numerical aperture, e.g. NA=0.85, are adopted to obtain a minute spot. With an increase in NA, however, the quality of the spot is noticeably reduced because of spherical aberration that occurs when laser light enters the cover layer of the disk and comatic aberration that occurs with tilt variation of the disk.
In order to reduce the spherical aberration and the comatic aberration, an extremely thin cover layer of 0.1 mm in thickness is adopted for the optical disk.
If there are thickness variations in a cover layer or among the cover layers of disks, the spot formed by transmitting laser light through the cover layer shows that spherical aberration occurs. The occurrence of the spherical aberration makes the spot blurry and larger in diameter and reduces central light intensity of the spot. The larger spot cannot read the minute signals accurately. In a case of the optical disk on a principal that a recording is performed by an optical heat, a decrease in the central light intensity results in its temperature not reaching a predetermined value required for the recording, thus not possible to record. On the other hand, if an entire amount of light is increased in order to obtain the predetermined temperature, an area above the predetermined temperature expands, thus not possible to record minutely.
In view of the above-mentioned problems, Japanese unexamined patent publication No. 367197/2002 A and others have proposed optical disk devices, which can accurately and reliably detect spherical aberration that results from thickness variations of the disk and misalignment in the optical system, and correct the spherical aberration.
By referring to drawings, descriptions will be made on such an optical disk device. FIG. 17 is a block diagram showing a basic structure of a conventional optical disk device that corrects spherical aberration. Light from a semiconductor laser 101 is collimated by a collimator lens 102, passes through a beam splitter 103 and is focused on a recording layer of an optical disk 108 through a cover layer by two objective lenses 106 and 107 in two units. A first lens 106 of the two objective lenses in two units is mounted in a bidirectional actuator 104 and driven in the direction of the optical axis and in the direction of radius of the optical disk. A second lens 107 is mounted on an actuator 105 for correcting spherical aberration, which is driven together with the first lens. These actuators 104 and 105 change the distance between the two lenses to generate spherical aberrations corresponding to the distance.
The light reflected off the optical disk 108 is reflected by the beam splitter 103 and enters a light-splitting hologram 109. Light around the optical axis and light at the periphery of the hologram 109 (not shown) are split toward different directions and enter a photodetector 112 through a cylindrical lens 111 by a condenser lens 110.
The photodetector 112 includes a plurality of light-receiving regions, which individually detect the light and convert the detected light into photoelectric currents. The photoelectric currents are output as a respective voltage signal from a focus error signal detector (AF) circuit 113, a tracking error signal detector (TR) circuit 114, a spherical aberration signal detector (SA) circuit 115 and a reproduction signal detector (RF) circuit 116.
The focus error signal is fed back to the bidirectional actuator 104 as a driving signal to move the bidirectional actuator 104 in the focusing direction (optical axis direction) so that an optimal light spot is constantly formed on the optical disk. The tracking error signal is fed back to the bidirectional actuator 104 as a driving signal to move the bidirectional actuator 104 in the direction of radius of the disk. The spherical aberration signal is fed back to the actuator 105 to control the actuator 105 to compensate spherical aberration, which results from the thickness deviation of the cover layer of the optical disk 108 or the improper distance between the lenses. The reproduction signal detector circuit 116 performs a series of processes for producing signals stored on the optical disk, including current-voltage conversion, waveform equalization and binarization.
By the way, a method for detecting the spherical aberration has been proposed, in which the focus error signals are detected individually from inner light and outer light of a light flux collected on the photodetector to obtain a differential signal between them. However, the focus error signal may be deteriorated by interference of light on the photodetector, thus the range in which spherical aberration can be detected stably may become narrower. In the above-mentioned device, prior to being collected onto the photodetector 112, the light flux is split by the light-splitting hologram 109 into inner light and outer light. The split lights are converged onto the different light-receiving regions of the photodetector 112; each region calculates a focus error signal to obtain their difference, thereby obtaining the spherical aberration signal. Accordingly, the spherical aberration signal is detected more reliably.
In the optical disk device disclosed in Japanese unexamined patent publication 367197/2002, the spherical aberration signal can be detected reliably. However, the device requires some extra detecting components including the light-splitting hologram and the light-receiving element with a special splitting pattern to detect spherical aberration, resulting in an increase in complexity and cost of the device.
Additionally, a liquid crystal element for correcting spherical aberration has been proposed recently. The liquid crystal element is used in an optical pick-up in which an isolator is composed of a polarizing beam splitter and a quarter wavelength plate to reduce noise generated by light returning to the laser. Detection of the spherical aberration with the use of such an optical pick-up comprising no specialized elements but the liquid crystal element entails following difficulties.
Prior to the description of the difficulties, a brief description about the structure and functions of the isolator will be made, which is well-known technology.
In FIG. 2, light emitted from a laser diode 26 is vertical (p-wave) linearly polarized light to a polarizing beam splitter 30. The light enters the polarizing beam splitter 30. The polarizing beam splitter 30 splits the vertical (p-wave) component of light into spectrum of transmitted light and reflected light at a predetermined ratio such as 9:1, while splitting the horizontal (s-wave) component of light into a spectrum of transmitted light and reflected light at a predetermined ratio such as 0:10, for example. In this case, one-tenth of the incident light is reflected by the polarizing beam splitter 30 and enters a front monitor 32, while remaining light is transmitted through the polarizing beam splitter 30.
The transmitted linearly polarized light being incident to a quarter (¼) wavelength plate 36 via a liquid crystal element 35 for correcting spherical aberration is converted into circularly polarized light. The light entering an objective lens 40 is focused on a signal surface of a disk 1 and then reflected off. The reflected light retraces the approaching route and passes through the quarter wavelength plate 36. In the quarter wavelength plate 36, the circularly polarized light is converted into linearly polarized light in opposite phase to the incoming light. In other words, the light has horizontal plane (s-wave) to the polarizing beam splitter 30. The light again enters the polarizing beam splitter 30 through the liquid crystal element 35 for correcting spherical aberration and is reflected 100% by the splitter 30 without returning to the laser 26.
Secondly, brief descriptions will be made about the liquid crystal elements 35 for correcting spherical aberration as a premise of the invention. The element comprises a plurality of separated electrodes that are concentric circles. By applying voltages individually to the separated electrodes, the liquid crystal element 35 is changed in its refraction index and provides phase difference to the light passing therethrough, thereby the spherical aberration is corrected. Because the spherical aberration caused by the thickness variation of the cover layer appears in the shape of a doughnut, the electrodes of the liquid crystal element 35 takes shape of circular bands that are concentric each other, and produce aberration (phase difference) opposite in shape to the spherical aberration generated at the disk. As a result, the aberrations are compensated for each other; therefore, the spherical aberration of the beam spot formed on the disk is compensated.
Problems associated with the liquid crystal element win be then described. In order to use the liquid crystal element, a quarter wavelength plate must be disposed downstream of the liquid crystal element on the approaching route. In this structure, reflected light from the disk passes through the quarter wavelength plate prior to the liquid crystal element on the returning route. The plane of the polarized light that enters the liquid crystal element on the returning route is perpendicular to the plane of the polarized light entering the element on the approaching route. Thus the liquid crystal element does not function to change the aberration and thus does not produce effect on wave front of the light. If the liquid crystal element works for light on both approaching and returning routes as normal lenses do, the collimated light entering the liquid crystal element on the approaching route will be collimated again by the liquid crystal element on the returning route and emitted from it. However, since the liquid crystal element does not produce effect on returning light, the light is not emitted as collimated light. Furthermore, the degree of convergence and diffusion of light on the returning route vary depending on the degree of the spherical aberration correction.
On the returning route, light is totally reflected by the polarizing beam splitter 30 and enters the light-receiving element to obtain various signals. The extent to which the spherical aberration is corrected by the liquid crystal element varies the extent to which light is converged onto the light-receiving element. This changes a focus error signal which drives a focus servo, therefore the light spot will be defocused on the disk.
Focusing light on the disk is the major premise to record and reproduce data on the optical disk. Without the premise, it is difficult to detect spherical aberration.
The present invention was made to solve the above-mentioned previously known problems and has an object to provide a device capable of correcting spherical aberration that results from difference of individual disk without extra detecting components.